From the second commit:
The surface offset allows an application to move itself in relative coordinates to its previous position. It is rather ill defined and partly incompatible with other functionality, which is why we ignore it generally. For dnd-surfaces though, it is the de-facto standard for applications to properly position the dnd-icon below the cursor. Therefore apply the offset on actor sync.
Fixes #679 (closed)
Tested with nautilus, weston-dnd and firefox (on X11) with and without (fractional) scaling. Firefox on Wayland with fractional scaling does net yet behave quite right, but I'm pretty sure that's a Firefox specific problem. When testing, have #685 (closed) in mind.
For Firefox/Thunderbird on HIDPI see https://bugzilla.mozilla.org/show_bug.cgi?id=1567463